<?php
/**
 * Class HTTP_ConditionalGet
 * @package Minify
 * @subpackage HTTP
 */

/**
 * Implement conditional GET via a timestamp or hash of content
 *
 * E.g. Content from DB with update time:
 * <code>
 * list($updateTime, $content) = getDbUpdateAndContent();
 * $cg = new HTTP_ConditionalGet(array(
 *     'lastModifiedTime' => $updateTime
 *     ,'isPublic' => true
 * ));
 * $cg->sendHeaders();
 * if ($cg->cacheIsValid) {
 *     exit();
 * }
 * echo $content;
 * </code>
 *
 * E.g. Shortcut for the above
 * <code>
 * HTTP_ConditionalGet::check($updateTime, true); // exits if client has cache
 * echo $content;
 * </code>
 *
 * E.g. Content from DB with no update time:
 * <code>
 * $content = getContentFromDB();
 * $cg = new HTTP_ConditionalGet(array(
 *     'contentHash' => md5($content)
 * ));
 * $cg->sendHeaders();
 * if ($cg->cacheIsValid) {
 *     exit();
 * }
 * echo $content;
 * </code>
 *
 * E.g. Static content with some static includes:
 * <code>
 * // before content
 * $cg = new HTTP_ConditionalGet(array(
 *     'lastUpdateTime' => max(
 *         filemtime(__FILE__)
 *         ,filemtime('/path/to/header.inc')
 *         ,filemtime('/path/to/footer.inc')
 *     )
 * ));
 * $cg->sendHeaders();
 * if ($cg->cacheIsValid) {
 *     exit();
 * }
 * </code>
 * @package Minify
 * @subpackage HTTP
 * @author Stephen Clay <steve@mrclay.org>
 */
class HTTP_ConditionalGet {

	/**
	 * Does the client have a valid copy of the requested resource?
	 *
	 * You'll want to check this after instantiating the object. If true, do
	 * not send content, just call sendHeaders() if you haven't already.
	 *
	 * @var bool
	 */
	public $cacheIsValid = null;

	/**
	 * @param array $spec options
	 *
	 * 'isPublic': (bool) if true, the Cache-Control header will contain
	 * "public", allowing proxies to cache the content. Otherwise "private" will
	 * be sent, allowing only browser caching. (default false)
	 *
	 * 'lastModifiedTime': (int) if given, both ETag AND Last-Modified headers
	 * will be sent with content. This is recommended.
	 *
	 * 'encoding': (string) if set, the header "Vary: Accept-Encoding" will
	 * always be sent and a truncated version of the encoding will be appended
	 * to the ETag. E.g. "pub123456;gz". This will also trigger a more lenient
	 * checking of the client's If-None-Match header, as the encoding portion of
	 * the ETag will be stripped before comparison.
	 *
	 * 'contentHash': (string) if given, only the ETag header can be sent with
	 * content (only HTTP1.1 clients can conditionally GET). The given string
	 * should be short with no quote characters and always change when the
	 * resource changes (recommend md5()). This is not needed/used if
	 * lastModifiedTime is given.
	 *
	 * 'eTag': (string) if given, this will be used as the ETag header rather
	 * than values based on lastModifiedTime or contentHash. Also the encoding
	 * string will not be appended to the given value as described above.
	 *
	 * 'invalidate': (bool) if true, the client cache will be considered invalid
	 * without testing. Effectively this disables conditional GET.
	 * (default false)
	 *
	 * 'maxAge': (int) if given, this will set the Cache-Control max-age in
	 * seconds, and also set the Expires header to the equivalent GMT date.
	 * After the max-age period has passed, the browser will again send a
	 * conditional GET to revalidate its cache.
	 *
	 * @return null
	 */
	public function __construct($spec)
	{
		$scope = (isset($spec['isPublic']) && $spec['isPublic'])
		? 'public'
		: 'private';
		$maxAge = 0;
		// backwards compatibility (can be removed later)
		if (isset($spec['setExpires'])
		&& is_numeric($spec['setExpires'])
		&& ! isset($spec['maxAge'])) {
			$spec['maxAge'] = $spec['setExpires'] - $_SERVER['REQUEST_TIME'];
		}
		if (isset($spec['maxAge'])) {
			$maxAge = $spec['maxAge'];
			$this->_headers['Expires'] = self::gmtDate(
			$_SERVER['REQUEST_TIME'] + $spec['maxAge']
			);
		}
		$etagAppend = '';
		if (isset($spec['encoding'])) {
			$this->_stripEtag = true;
			$this->_headers['Vary'] = 'Accept-Encoding';
			if ('' !== $spec['encoding']) {
				if (0 === strpos($spec['encoding'], 'x-')) {
					$spec['encoding'] = substr($spec['encoding'], 2);
				}
				$etagAppend = ';' . substr($spec['encoding'], 0, 2);
			}
		}
		if (isset($spec['lastModifiedTime'])) {
			$this->_setLastModified($spec['lastModifiedTime']);
			if (isset($spec['eTag'])) { // Use it
				$this->_setEtag($spec['eTag'], $scope);
			} else { // base both headers on time
				$this->_setEtag($spec['lastModifiedTime'] . $etagAppend, $scope);
			}
		} elseif (isset($spec['eTag'])) { // Use it
			$this->_setEtag($spec['eTag'], $scope);
		} elseif (isset($spec['contentHash'])) { // Use the hash as the ETag
			$this->_setEtag($spec['contentHash'] . $etagAppend, $scope);
		}
		$this->_headers['Cache-Control'] = "max-age={$maxAge}, {$scope}";
		// invalidate cache if disabled, otherwise check
		$this->cacheIsValid = (isset($spec['invalidate']) && $spec['invalidate'])
		? false
		: $this->_isCacheValid();
	}

	/**
	 * Get array of output headers to be sent
	 *
	 * In the case of 304 responses, this array will only contain the response
	 * code header: array('_responseCode' => 'HTTP/1.0 304 Not Modified')
	 *
	 * Otherwise something like:
	 * <code>
	 * array(
	 *     'Cache-Control' => 'max-age=0, public'
	 *     ,'ETag' => '"foobar"'
	 * )
	 * </code>
	 *
	 * @return array
	 */
	public function getHeaders()
	{
		return $this->_headers;
	}

	/**
	 * Set the Content-Length header in bytes
	 *
	 * With most PHP configs, as long as you don't flush() output, this method
	 * is not needed and PHP will buffer all output and set Content-Length for
	 * you. Otherwise you'll want to call this to let the client know up front.
	 *
	 * @param int $bytes
	 *
	 * @return int copy of input $bytes
	 */
	public function setContentLength($bytes)
	{
		return $this->_headers['Content-Length'] = $bytes;
	}

	/**
	 * Send headers
	 *
	 * @see getHeaders()
	 *
	 * Note this doesn't "clear" the headers. Calling sendHeaders() will
	 * call header() again (but probably have not effect) and getHeaders() will
	 * still return the headers.
	 *
	 * @return null
	 */
	public function sendHeaders()
	{
		$headers = $this->_headers;
		if (array_key_exists('_responseCode', $headers)) {
			header($headers['_responseCode']);
			unset($headers['_responseCode']);
		}
		foreach ($headers as $name => $val) {
			header($name . ': ' . $val);
		}
	}

	/**
	 * Exit if the client's cache is valid for this resource
	 *
	 * This is a convenience method for common use of the class
	 *
	 * @param int $lastModifiedTime if given, both ETag AND Last-Modified headers
	 * will be sent with content. This is recommended.
	 *
	 * @param bool $isPublic (default false) if true, the Cache-Control header
	 * will contain "public", allowing proxies to cache the content. Otherwise
	 * "private" will be sent, allowing only browser caching.
	 *
	 * @param array $options (default empty) additional options for constructor
	 *
	 * @return null
	 */
	public static function check($lastModifiedTime = null, $isPublic = false, $options = array())
	{
		if (null !== $lastModifiedTime) {
			$options['lastModifiedTime'] = (int)$lastModifiedTime;
		}
		$options['isPublic'] = (bool)$isPublic;
		$cg = new HTTP_ConditionalGet($options);
		$cg->sendHeaders();
		if ($cg->cacheIsValid) {
			exit();
		}
	}


	/**
	 * Get a GMT formatted date for use in HTTP headers
	 *
	 * <code>
	 * header('Expires: ' . HTTP_ConditionalGet::gmtdate($time));
	 * </code>
	 *
	 * @param int $time unix timestamp
	 *
	 * @return string
	 */
	public static function gmtDate($time)
	{
		return gmdate('D, d M Y H:i:s \G\M\T', $time);
	}

	protected $_headers = array();
	protected $_lmTime = null;
	protected $_etag = null;
	protected $_stripEtag = false;

	protected function _setEtag($hash, $scope)
	{
		$this->_etag = '"' . substr($scope, 0, 3) . $hash . '"';
		$this->_headers['ETag'] = $this->_etag;
	}

	protected function _setLastModified($time)
	{
		$this->_lmTime = (int)$time;
		$this->_headers['Last-Modified'] = self::gmtDate($time);
	}

	/**
	 * Determine validity of client cache and queue 304 header if valid
	 */
	protected function _isCacheValid()
	{
		if (null === $this->_etag) {
			// lmTime is copied to ETag, so this condition implies that the
			// server sent neither ETag nor Last-Modified, so the client can't
			// possibly has a valid cache.
			return false;
		}
		$isValid = ($this->resourceMatchedEtag() || $this->resourceNotModified());
		if ($isValid) {
			$this->_headers['_responseCode'] = 'HTTP/1.0 304 Not Modified';
		}
		return $isValid;
	}

	protected function resourceMatchedEtag()
	{
		if (!isset($_SERVER['HTTP_IF_NONE_MATCH'])) {
			return false;
		}
		$clientEtagList = get_magic_quotes_gpc()
		? stripslashes($_SERVER['HTTP_IF_NONE_MATCH'])
		: $_SERVER['HTTP_IF_NONE_MATCH'];
		$clientEtags = explode(',', $clientEtagList);

		$compareTo = $this->normalizeEtag($this->_etag);
		foreach ($clientEtags as $clientEtag) {
			if ($this->normalizeEtag($clientEtag) === $compareTo) {
				// respond with the client's matched ETag, even if it's not what
				// we would've sent by default
				$this->_headers['ETag'] = trim($clientEtag);
				return true;
			}
		}
		return false;
	}

	protected function normalizeEtag($etag) {
		$etag = trim($etag);
		return $this->_stripEtag
		? preg_replace('/;\\w\\w"$/', '"', $etag)
		: $etag;
	}

	protected function resourceNotModified()
	{
		if (!isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
			return false;
		}
		$ifModifiedSince = $_SERVER['HTTP_IF_MODIFIED_SINCE'];
		if (false !== ($semicolon = strrpos($ifModifiedSince, ';'))) {
			// IE has tacked on extra data to this header, strip it
			$ifModifiedSince = substr($ifModifiedSince, 0, $semicolon);
		}
		if ($ifModifiedSince == self::gmtDate($this->_lmTime)) {
			// Apache 2.2's behavior. If there was no ETag match, send the
			// non-encoded version of the ETag value.
			$this->_headers['ETag'] = $this->normalizeEtag($this->_etag);
			return true;
		}
		return false;
	}
}
